1. Field of the Invention
This invention relates to a method for treating sulfur free-cutting alloy steel used in the production of precision machine parts and, more particularly, relates to a method for treating sulfur free-cutting alloy steel to remove fine granular sulfide of Mn, Cr, Mo, etc. left on a machined surface of sulfur free-cutting alloy steel.
2. Prior Art
A machined surface of sulfur free-cutting alloy steel has been conventionally cleaned by applying ultrasonic waves of 0.3 to 0.9 W/cm2 in watt density for 5 to 10 minutes without specifying frequency. Such ultrasonic cleaning has been effectively applied to usual precision machining and hardly caused troubles.
However, it has been found in the case of ultra-precision machine such as bearings for driving a small hard disk of superior high density, that fine granular sulfides of Mn, Cr, Mo, etc. (Mn—S, Cr—S, Mo—S, etc.) left on a machined surface of sulfur free-cutting alloy steel happens to drop out from the surface, thereby troublesomely crushing a disk or head of a hard disk drive (HDD).
The inventors have tried to remove or decrease such fine granular sulfides of Mn, Cr, Mo, etc. (Mn—S, Cr—S, Mo—S, etc.) left on the machined surface without affecting the form and dimensional accuracy when sulfur free-cutting alloy steel is machined into parts of a predetermined form and dimension.
As a result of the inventors' eager investigation, it has been found that an unexpected effect of ultrasonic cleaning is achieved by treating the surface with ultrasonic waves of 1.0 to 2.0 W/cm2 in watt density at a frequency of 40 kHz or multi-frequency for 5 to 10 minutes in order to prevent drop-out of the fine granular sulfide of Mn, Cr, Mo, etc. (Mn—S, Cr—S, Mo—S, etc.) left on the machined surface of sulfur free-cutting alloy steel. The present invention is developed on the basis of this information.